A fire apparently set by an employee at a Chicago-area air traffic control center led to the cancelation of more than 1,700 flights at the city's two major airports, snarling air traffic across the United States, officials said.
Flights resumed at O'Hare International Airport, one of the world's busiest, after a delay of about four hours, but there were bottlenecks across the entire air system that carriers expect to last through the weekend.
There were no indications that the fire was an act of terrorism and the blaze was quickly extinguished, authorities said. The Federal Aviation Administration said employees were evacuated from its control center when the fire broke out. One person was treated for smoke inhalation.